Xerographic development electrode apparatus



Oct. 31, 1967 HUD N 3,349,676

XEROGRAPHIC DEVELOPMENT ELECTRODE APPARATUS Filed April 2, 1965 INVENTOR. FREDERICK W. HUDSON A TTORNE Y5 United States Patent Filed Apr. 2, 1965, Ser. No. 445,133 5 Claims. (Cl. 95-1.7)

This invention relates to development electrode apparatus for developing electrostatic latent images formed on a continuously advancing xerographic plate such as a xerographic drum mounted for rotation.

In the process of xerography, for example, as disclosed in Carlson Patent 2,297,691, issued Oct. 6, 1942, a xerographic plate comprising a layer of photoconductive insulating material on a conductive backing is given a uniform electric charge over its surface and is then exposed to the subject matter to be reproduced, usually by conventional projection techniques. This exposure discharges the plate areas in accordance with the radiation intensity that reaches them and thereby creates an electrostatic latent image on or in the photo conductive layer. Development of the latent image is effected with an electrostatically charged, finely divided material such as an electroscopic powder that is brought into surface contact with the photo conductive layer and is held thereon electrostatically in a pattern corresponding to the electrostatic latent image. Thereafter, the developed xerographic powder image is usually transferred to a support surface to which it may be fixed by any suitable means.

Many development systems for developing electrostatic latent images are known in the xerographic art including brush, magnetic brush, liquid and cascade to name a few. The most commonly employed of the above that enjoys by far the widest commercial utility is the cascade system in which a two-component developer composition, as for example, disclosed in patent US. 2,618,- 552 is caused to cascade over the charge bearing surface of the xerographic plate to effect development of the image thereon. The two-components of the cascade developer include a relatively hard carrier bead which delivers and triboelectrically charges the relatively smaller releasable electroscopic powder termed toner of generally resinous composition, as for example, disclosed in Reissue U.S. 25,136. During the cascading of the developer composition, the toner is attracted from the carrier to the latent image to effect development of the image while denuding the carrier.

In the usual line-copy form of development, such as black letters on a white background, the developer powder in cascading over the plate surface adheres to the electrostatic latent image with sharpness and uniform density. However, Where the copy to be reproduced is possessed of images having at least some areas of other than linecopy form such as solidly wide, dark areas termed solid area, the cascade development technique is by itself known to be inherently unable to effect faithful development thereof. Instead of a faithful uniform deposition across the entire image area, there results a halo appearing development consisting of developed fringes about the image perimeter and an absence of developer in the central portions, owing to the improper and non-uniform adherence of the developer powder thereat. This effect is believed caused by the characteristics of the charge forces associated with the central portions of the solid area. Rather than the electrostatic lines of force being directed outwardly perpendicular to the plate surface, they are directed instead in curved paths to the edges or outer portions thereof into the adjacent discharged or background areas. This renders the force lines of insufficient magnitude to attract the developer powder from the carrier and results in the central portions being vacant of developer.

The use of development electrodes in order to at least partially alleviate this difficulty is known in the art and apparatus therefor is described, for example, in patents US 2,777,418 and 3,011,474. The development electrode, as is known in the prior art, is closely supported opposite the image surface over which the developer is cascaded. In this position, it serves to effect some correction to the electrical field existing in the central portions of solid areas by causing the lines of force emanating from these areas to be directed in generally parallel relation towards the surface of the electrode. This has been known to enhance deposition of the developer powder over these image areas to produce more uniformly developed images corresponding more nearly to the original of which the reproduction is to represent.

Despite the improvements afforded by these prior art electrode devices, it has not been heretofore possible to exploit the benefits thereof to the maximum. The affect of an electrode is known to decrease as its spacing from the image surface increases and contra, minimum spacing affords maximum benefit. Because the carrier particles employed in the cascade development system are of a hard, granular material, as compared to a relatively soft, easily destroyable photoconductive material comprising the xerographic plate, it has always been necessary with prior art apparatus to support the electrode spaced apart from the plate surface sufficient to permit freedom of developer flow without binding. Accordingly, because of a practical limitation it has not heretofore been possible to maintain the electrode at a minimum spacing at which optimum results can be obtained. That is, in acontinuously operative system with the developer composition cascading over the photoconductive surface of a moving xerographic plate relative to a stationary electrode, it has been necessary to comprise the electrode to drum spacing in order to prevent any binding of the carrier particles therebetween which might likely score or otherwise deleteriously affect the relatively soft surface of the photoconductor. Spacing therefore, has been critical and difficult to maintain particularly as associated with rotating drum type xerographic plates with which electrode curvature had to be accurately maintained in order to prevent local ized binding. As a result these prior art devices, while improving development characteristics as compared to development with an absence hereof, have never fully realized maximum benefits that could be obtained with much closer spacings.

Now in accordance with the instant invention, there has been discovered novel development electrode apparatus by which the benefits afforded by the electrode field correction can be obtained to a maximum. This is achieved in accordance herewith by means of a plurality of loosely hanging conductive chains controllably spaced and suspended freely on the developer stream during the cascadement thereof. Whereas it might be expected that an electrode of this type might likely discharge the image charges as to destroy the image effect, it has to the contrary been found that with controlled arrangements of the freely suspended chain members and developer flow, that the chain members ride directly on the developer surface without deleteriously affecting the image charges below.

It is therefore an object of the invention to provide novel development electrode apparatus for developing electrostatic latent images on a continuously movable xerographic plate.

It is a further object of the invention to provide novel development electrode apparatus able in an automatic xerographic machine to obtain substantially greater correction of charge forces associated with solid areas than has been possible by the devices of the prior art.

It is a still further object of the invention to provide novel development electrode apparatus simpler to construct and utilize and affording enhanced benefit with automatic drum type xerographic equipment than devices of the prior art.

These and other objects of the invention are achieved in accordance with an embodiment of the invention which is exemplified by the following drawings in which:

FIG. 1 schematically illustrates an automatic xerographic apparatus employing the development electrode device hereof; and

FIG. 2 isometrically illustrates an enlarged view of the development electrode apparatus as employed in FIG. 1.

For a general understanding of the xerographic processing system in which the invention is incorporated, reference is had to FIG. 1 in which the various system components are schematically illustrated. As in all xerographic systems based on the concept disclosed in the above-cited Carlson patent, a radiation light image of copy to be reproduced is projected onto the sensitized surface of a xerographic plate to form an electrostatic latent image thereon. Thereafter, the latent image is usually developed with an oppositely charged developing material to form a xerographic powder image, corresonding to the latent image, on the plate surface. The powder image is then electrostatically transferred to a support surface to which it may be fused by any suitable form of fusing device, whereby the powder image is caused permanently to adhere to the support surface.

In the apparatus illustrated, an original copy to be reproduced is placed on a support tray from which it is fed onto a transport mechanism generally designated 11. Suitable drive means are provided for the transport mechanism from motor 12 to endless belts 13 whereby the copy is moved past the optical axis of projection lens system 14 that is illuminated by a projection lamp LMP-1. The image of the copy is reflected by mirror 15 through an adjustable objective lens 16 and then reflected by mirror 17 downwardly through a variable slit aperture assembly 18 and onto the surface of a xerographic plate in the form of drum 19.

Xerographic drum 19 includes a cylindrical member mounted in suitable bearings in the frame of the machine and is driven in a clockwise direction by a motor 24 at a constant rate that is proportional to the transport rate of the copy, whereby the peripheral rate of the drum surface is identical to the rate of movement of the reflected light image. The drum surface comprises a layer of photoconductive material on a conducting backing that is sensitized to about 300-1000 volts prior to exposure by means of a corona generating device 25, which may be an adaptation of the type disclosed in patent U.S. 2,836,725 that is energized from a suitable high potential source.

Exposure of the drum to the light image discharges the photoconductive layer in the areas struck by light, whereby there remains on the drum a latent electrostatic image in image configuration corresponding to the light image projected from the copy. As the drum surface continues its movement, the electrostatic latent image passes through a developing station 26 at which a twocomponent cascade developer material 27, as described above, is cascaded over the drum surface by means of developing apparatus 28. As will be described below, the development electrode device 23 of the invention is supported in contact with the developer stream as it cascades over the image surface.

In the developing apparatus, developer material is carried up by conveyor 29 driven by suitable drive means from motor 30 and is released onto chute 31 wherefrom it cascades down over the drum surface. Toner compo nent 32 of the developer that is consumed in developing is stored in dispenser 33 and is released in controlled quantities by gate 34.

After developing, the Xerographic powder image passes a discharge station 41 at which the drum surface is illumi nated by a lamp LMP-Z, whereby residual charges on the non-image areas of the drum surface are completely discharged. Thereafter, the powder image passes through an image transfer station 42 at which the powder image is electrostatically transferred to a support surface web 43 by means of a second corona generating device 44 similar to corona generating device 25, mentioned above.

The support surface to which the powder image is transferred may be of any convenient type such as paper and is obtained from a supply roll 45 and is fed over guide rolls 46 and 47 being directed into surface contact with the drum in the immediate vicinity of transfer generating device 44. After transfer, the support surface is separated from the drum surface and is guided through a suitable fusing apparatus 48 whereat the powder image is permanently affixed to the support surface. Thereafter, the support surface is fed over a further sfysteni of guide and tensioning rolls and onto a takeup roll 52 that is driven by motor 53.

After separation of the support surface from the drum, a corona generating device 54 directs electrostatic charge to the residual powder image on the drum surface to facilitate removal of the powder therefrom. The xerographic drum then passes through a cleaning station 55 at which the surface is brushed by a cleaning brush assembly 56, rotated by a motor 57, whereby residual developing material remaining on the drum is removed. Thereafter, the drum surface passes through a second discharge station 58 at which it is illuminated by a fluorescent lamp LMP3, whereby the drum surface in this region is completely flooded with light to remove any electrostatic charge that may remain thereon. Suitable light traps are provided in the system to prevent any light rays from reaching the drum surface, other than the projected image, during the period of drum travel immediately prior to sensitization by corona generating device 25 until after the drum surface is completely passed through the developing station 26.

The development electrode device of the invention will now be described with reference also to FIG. 2. The device includes a grounded conductive support rod 70 secured juxtaposed along the lower end of chute 31 and extending axially parallel to drum 19 laterally across the peripheral surface thereof. The rod is formed of continuous rod stock having a plurality of uniformly formed spaced apart recesses 71 from which are suspendedly attached freely hanging, flexible and relaxed elongated electrode members 72. These members are adapted to ride in contact with the developer and are closely spaced as will be described to collectively form a blanket of elongated electrodes opposite the imagebearing surface of the drum. Each of the electrode members 72 can comprise any suitable electrically conductive material generally relaxed when suspended freely from the attached support as to closely follow the contour of the developer stream on which it is supported. Found particularly suitable for this purpose and cited by way of example only are link-type bead chains 40-mil diameter sterling silver and SO-mil diameter brass. Smaller dimensional chains down to about mils are likewise operable as are chains up to approximately 100 mils. The chain dimensions are not regarded as critical except to the extent that they must be supportable by the cascading developer out of contact with the photoconductor surface.

As the chains rest on the passing developer a certain amount of drag is imposed on the chains as a function of the developer flow characteristics. In order therefore to be operable, the edge-to-edge spacing between adjacent hanging chains should be sufiicient so as not to cause a discontinuity in the electrode effect and which has been found to occur at spacings of about 100 mils and above. At spacings of approximately above about 100 mils with the above chain sizes the developer bed appears unable to support the chains which fall through the stream to contact the photoconductive surface. Electrode length is not critical but should generally approximate at least the length of the peripheral contact by the developer.

In order to obtain optimum uniform results herewith whereby the developer bed provides adequate electrode support, it was found preferable to control the rate of developer flow from chute 31. This is accomplished by means of a gate 73 extending through a slot 74 in the front wall of the developer housing partially across the opening 75 from which the developer emerges on the chute 31. By appropriate positioning of gate 73, any desired opening dimension can be maintained. Developer quantities of about 2 /2 to 3 pounds per minute per inch of drum width forming a developer depth of about inch was found to produce high quality reproductions. This compared to to 12 pounds per minute employed with prior art techniques.

As the developer cascades out from the chute, the electrode members seat themselves Within the developer stream in contact thereon and to some extent are dragged forwardly by the momentum of the passing developer. Being completely relaxed in contact with the developer, they individually follow the flow contour thereof as to be continuously spaced closely and uniformly from the drum surface. With the closeness of spacing between the electrodes and drum surface, maximum possible field correction is afforded the central portions of solid area development. As stated above, such a result is not attainable with prior art devices. Highly satisfactory results have been obtained with this device for drum speeds from approximately 0.5 to approximately 8 inches per second.

By the above description, there has been disclosed novel development electrode apparatus consisting of a plurality of closely spaced, freely hanging, grounded electrode members supported directly in contact with the cascade developer as it passes over the xerographic drum to effect development of latent images thereon. Since the electrode members are in direct contact with the developer, an absolute minimum spacing is effected for controlling the charge forces associated with the image characters, particularly as associated with the central portions of solid areas for enhancing the development thereof. The device while being generally simpler and more easily fabricated than devices of the prior art, has also been found to produce print quality generally superior to that obtained with prior art devices. While described in connection with specific embodiments, certain variations can readily be adapted therein. For example, instead of grounding the electrodes, they can where desired, be biased to a fixed potential generally greater than the potential retained in the background non-image areas of the xerographic plate as to suppress development thereat. Whereas particular parameters have been given with regard to electrode type, size and spacing these are not to be regarded as limitations. Rather, they have been found to vary in accordance with the particular developer composition, rate of drum speed and the like. With increased drum speeds, the electrode spacing would not necessarily be affected, whereas developer flow rate,

6 relaxed electrode weight, etc. would likely bear on optimum parameters.

Since many changes could be made in the above construction and many apparently widely different embodiment of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the drawings and specification shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a xerographic reproduction unit including a xerographic plate, means to form an electrostatic latent image of copy on the surface of said plate and developing apparatus for delivering two-component cascade developer to the plate surface for cascade development of the latent image thereon, a development electrode apparatus comprising in combination:

(a) a plurality of flexible, elongated electrodes;

(b) means to support said electrodes uniformly spaced apart and freely suspended substantially parallel in overlying contact with the developer stream cascading on the surface of said plate; and,

(0) means to connect said eelctrodes to a reference potential.

2. In a xerographic reproduction unit including a continuously rotating xerographic drum, means to form an electrostatic latent image of copy on the surface of said drum and developing apparatus for delivering two-component cascade developer to the drum surface for cascade development of the latent image thereon, a development electrode apparatus comprising in combination:

(a) a plurality of flexible, elongated electrodes;

(b) means to support said electrodes uniformly spaced apart and freely suspended substantially parallel in overlying contact with the developer stream cascading on the surface of said drum; and,

(c) means to connect said electrodes to a reference potential.

3. A xerographic reproduction unit comprising in combination:

(a) means to support a xerographic plate;

(b) means to form an electrostatic latent image of copy on the surface of said plate; and,

(0) developing apparatus delivering two-component cascade developer to the plate surface for cascade development of the latent image thereon said developing apparatus including a development electrode apparatus comprising in combination:

(1) a plurality of flexible, elongated electrodes;

(2) means to support said electrodes uniformly spaced apart and freely suspended substantially parallel in overlying contact with the developer stream cascading on the surface of said plate; and

(3) means to connect said electrodes to a reference potential.

4. In a xerographic reproduction unit including a continuously rotating xerographic drum, means to form an electrostatic latent image of copy on the surface of said drum and developing apparatus for delivering two-component cascade developer to the drum surface for cascade development of the latent image thereon, a development electrode apparatus comprising in combination:

(a) a plurality of relaxed elongated electrodes comprising loosely hanging electrically conductive chains;

(b) means to support said electrodes uniformly spaced apart and freely suspended substantially parallel in overlying contact with the developer stream cascading on the surface of said drum; and

(c) means to connect said electrodes to a reference potential.

5. In a xerographic reproduction unit including a continuously rotating xerographic drum, means to form an electrostatic latent image of copy on the surface of said drum and developing apparatus for delivering two-component cascade developer to the drum surface for cascade 7 development of the latent image thereon, a development electrode apparatus comprising in combination:

(a) a plurality of relaxed elongated electrodes comprising loosely hanging electrically conductive chains of the bead and socket type and having beads of up tQ 80-rnils in diameter;

(b) means to support said electrodes uniformly spaced I apart transversely relative to said drum surface a distance of approximately up to about lQO-rnils and freely suspended substantially parallel in overlying contact with the developer stream cascading on the surface of said drum; and,

(0) means to connect said electrodes to a reference potential.

References Cited UNITED STATES PATENTS 2,573,881 11/1951 Walkup et al. 95-17 X 2,952,241 9/1960 Clark et a1. 118--637 2,965,573 12/1960 Gundlach 117 17.5 X 3,011,474 12/1961 Ulrich 95,1.7X 3,147,147 9/1964 Carlson 95 1.7 X

JOHN M. HORAN, Primary Examiner. EVON C. BLUNK, Examiner. 

1. IN A XEROGRAPHIC REPRODUCTION UNIT INCLUDING A XEROGRAPHIC PLATE, MEANS TO FORM AN ELECTROSTATIC LATENT IMAGE OF COPY ON THE SURFACE OF SAID PLATE AND DEVELOPING APPARTUS FOR DELIVERING TWO-COMPONENT CASCADE DEVELOPER TO THE PLATE SURFACE FOR CASCADE DEVELOPMENT OF THE LATENT IMAGE THEREON, A DEVELOPMENT ELECTRODE APPARATUS COMPRISING IN COMBINATION: (A) A PLURALITY OF FLEXIBLE, ELONGATED ELECTRODES; (B) MEANS TO SUPPORT SAID ELECTRODE UNIFORMLY SPACED APART AND FREELY SUSPENDED SUBSTANTIALLY PARALLEL IN OVERLYING CONTACT WITH THE DEVELOPER STREAM CASCADING ON THE SURFACE OF SAID PLATE; AND, (C) MEANS TO CONNECT SAID ELECTRODES TO A REFERENCE POTENTIAL. 